1. Field of the Invention
This invention relates to a liquid-solids slurry filter, more particularly such a filter of the type wherein a cake of solids deposited from the slurry on the filter medium, is subjected to one or more treatments, such as washing, involving the passage of liquid through the cake while it is retained on the filter medium.
2. Description of the Prior Art
Filters of the type concerned are typified by so-called horizontal rotary pan filters and belt filters. Filters of the first type have a horizontally disposed, generally circular array of pans with filters at their bottoms. Slurry fed to each pan is formed into a cake on each bottom filter by suction applied thereunder, wash water is applied to the cake and drawn through it by suction, usually several times, and the washed cake is finally removed, either by tilting the pan to dump it, or by moving scrapers. The array of pans is usually rotated about a vertical axis past the various feed and treating stations (see e.g. U.S. Pat. No. 2,684,158), but the pans may be stationary and the feed and wash equipment rotated about such axis (see e.g. U.S. Pat. No. 2,687,806).
In belt filters an endless belt carries the slurry generally horizontally over suction boxes from a feed station past cake wash and drying treatment stations. The belt may carry pans with filter bottoms (see e.g. U.S. Pat. No. 2,677,467) or the belt may constitute or comprise the filter medium, being formed to contain the slurry and cake through the aforesaid stages as the belt passes over the vacuum pans, which may be stationary (see e.g. U.S. Pat. No. 2,034,784), or may reciprocate along the belt path (see e.g. U.S. Pat. No. 2,997,179).
In utilizing such filters with many types of slurries which contain both fine and coarse particles difficulty is experienced in forcing the wash water through the formed cake, even with strong suction force applied. Pretreating the slurry as in centrifugal equipment to remove unwanted fines or "slimes" alleviates the problem to some extent but not fully, and slow drainage rate of cakes continues to slow the processing and production rates as well as producing wetter cakes than desired.